The manufacturing process of inner liners and door liners of refrigerating appliances usually comprises the process of sheet extrusion and one or several thermoforming phases. The plastic material often used is basically styrenics, for instance high impact polystyrene, polystyrene, ABS, which is extremely easy to process by extrusion and by thermoforming. The reason for the easy performance in processing the material is its amorphous nature.
Alternative sheet materials are polypropylene or other semi-crystalline polymers. Due to their chemical nature, however, which is inherently semi-crystalline, it is difficult to thermoform big and/or spatially extended parts made of these materials having high stretching ratios such as inner liners. The constraints on the desired shapes limit the “drop-in” replacement of styrenics by polypropylene. Commercial grades of polypropylene suitable for highly stretched parts are not currently available.
The current production of liners usually comprises as a first step a sheet extrusion with current extrusion rates of 500 kg/h-1,500 kg/h. The sheets are then cut in suitable dimension. The production process involves batches of hundreds of sheets. The batches are stored in warehouses to cool down for usually five days. After they have sufficiently cooled down, the sheets are thermoformed in order to get inner liners or door liners. The duration of a thermoforming cycle per part is typically between 30 s and 45 s.
A method for manufacturing refrigerator liners from crystalline synthetic resins is disclosed in U.S. Pat. No. 4,842,742, whereby polypropylene is used for sheets. The method described therein is called hybrid forming process, involving solid phase vacuum, stretch and pressure forming techniques. The polypropylene material is extruded and then cut into suitable billets. These billets are allowed to cool down for a short time before they are delivered to a forming station. A disadvantage of the described method is the operating expense due to the employment of several methods.
A method for making sheets and articles thermoformed is disclosed in WO 93/21262, whereby this method involves thermoforming a resinous polymer of propylene-containing sheet with steps of melt forming, quenching, heating, and thermoforming. It has therein been found that an effective amount of beta-spherulite nucleating agent is useful for preparing a thermoformable sheet. A disadvantage of this method lies in the requirement of the presence of a special agent in the material which creates extra production costs.
The object of the present invention is to provide a manufacturing method for liners, especially for inner liners for refrigerators, which avoids the vacuum forming step or deep thermoforming step.
Another object of the invention is to provide a method for manufacturing liners with efficient production rates and allowing for non-amorphous materials.
Still another object of the invention is to provide a liner that can be produced in an efficient and time-saving process.
Yet another object of the invention is to provide a household appliance with such a liner.